Rendering devices such as printers, copiers and the like, can employ incremental encoders to track the position of moving components such as print drums, rotating shafts, and print heads. The position or angle of these moving components is typically controlled by a drive mechanism and measured by the encoder. The incremental encoder typically includes a movable code wheel or code strip, with an optical track comprised of alternating transparent and opaque bars, that passes between and moves relative to an illumination source, typically a light emitting diode (LED) and sensor array composed of a plurality of photosensitive elements usually photodiodes.
As the code wheel/strip moves, it interrupts the light from the illumination source causing an alternating series of light and shadow to pass across the sensor array producing electric signals that vary in amplitude. Circuitry in the encoder reader amplifies the signals to produce two output signals in quadrature, phase-A and phase-B, which is phase shifted from phase-A by 90 electrical degrees. As the code wheel/strip moves, this quadrature signal varies in frequency proportional to the speed of motion, and the phase relationship between phase-A and phase-B indicates the direction of motion.
Since the system was powered-up or reset, the basic incremental reader determined two outputs that are in quadrature only relative angle or position, but not an absolute position. An additional method must be provided to determine an absolute reference position after power-up or reset. In the case of linear motion (code strip), stalling the drive mechanism into a mechanical hard stop is a common method for generating an absolute reference position (also called a “home” position). The incremental quadrature encoder is then utilized to measure position changes relative to this reference position.
Encoders measuring the angular position of a shaft typically need a different method for establishing an absolute reference (home) position, as a mechanical stop is more difficult to implement on a rotating shaft. The common solution is a third encoder sensor output reading a second optical track on the code wheel at a different radius than the primary optical track. Such second optical tracks can consist of a single dark or pattern of dark and light bars at the reference (home) position. This third encoder sensor output is also utilized in linear (code strip) applications where mechanical stalling is not desirable. The third channel technique adds the cost of a third sense channel to the encoder reader and it's associated cabling. Based on the foregoing it is believed that a need exists for improved system and method for sensing encoder home position.